The government is to raise the retirement age for public sector workers to 65, in a move that will set it on a collision course with unions.

In an interview with The Observer, Trade and Industry secretary Alan Johnson said the case for a rise in the retirement age was 'irrefutable' and that he was determined to press ahead with the change. His remarks come two days ahead of a crunch meeting with unions to discuss options for changing the system to address demographic change and the financial burden.

In March, John Prescott retreated from imposing an increase in the retirement age in the face of widespread public sector strikes. Last month, faced with similar threats, Johnson headed off a potentially explosive confrontation at the Trades Unions Congress in a speech that avoided committing to a change in the retirement age. A meeting of the government/union Public Sector Forum was scheduled for the Labour Party conference, but this was postponed until Tuesday.

'In March we said that the unions had a fair point when they said we were not negotiating properly,' Johnson said. 'If we wanted to move to normalise pension age from 60 to 65 we were obliged to make the argument. That argument is irrefutable. It is demographic change - we are healthier, living longer. The problem is there are fewer people working funding more people in retirement. For us to say to the private sector you have to work longer and save more money, and to the public sector you stick with your retirement age is impossible.'

He said the government would maintain final salary schemes, but that 'this does not make us any less convinced that we are going to move from 60 to 65'. Johnson is optimistic about avoiding widespread strikes.

The government aims to allow those who wish to retire early to do so, choosing either to pay more into their funds or to accept lower benefits.

Dave Prentis of Unison said: 'There is no point saying we did not negotiate on this properly, then saying there is a part of it that is non-negotiable.' He added that increases in life expectancy for public sector workers did not match the population as a whole.

PCS general secretary Mark Serwotka said: 'If they press for this, the threats for strike action will become a reality.'